The present invention relates to a process for producing a polyester. More particularly, the invention relates to a process for producing a polyester, comprising a step of performing a ring-opening copolymerization reaction of a cyclic acid anhydride containing succinic anhydride as a main component with a cyclic ether containing ethylene oxide as a main component.
An aliphatic polyester has biodegradability and is used for a packing material and the like by combining one or more kinds of additives with the polyester to mold the resulting composition into a sheet or film shape. As a process for producing such a polyester, there is generally performed a process comprising esterifying directly a dicarboxylic acid and a glycol or performing a transesterification reaction between an alkyl ester of a dicarboxylic acid and a glycol to produce a glycol ester and/or an oligomer thereof and then heating the glycol ester and/or oligomer thereof with stirring Under a high vacuum for a long period of time to perform a polycondensation reaction.
However, the process, which comprises performing a polycondensation reaction by heating with stirring under a high vacuum for a long period of time, needs a vacuum apparatus and high power to maintain a high vacuum. Therefore, efficiency is not industrially good.
Japanese Official Patent Gazette No. Showa 42-26708 has proposed a process for producing a polyester without using the vacuum apparatus and high power to maintain a high vacuum. The proposed process for producing a polyester comprises performing a copolymerization reaction of an alkylene oxide with a cyclic acid anhydride by using a catalyst system comprising as one component an organic compound of a metal selected from groups I to III in a periodic law table to form a polyester. In Examples 1 to 6 of the Gazette, an alkylene oxide, a cyclic acid anhydride, a solvent and a catalyst are placed and dissolved in a polymerization tube (a tube for polymerization), this tube is sealed, and a copolymerization reaction is performed in the sealed tube at 80.degree. C. (Examples 1 to 4 and 6) or 30.degree. C. (Example 5). An inside pressure of the sealed tube is higher than atmospheric pressure owing to a nitrogen gas and vapor of the solvent. The alkylene oxides used are epichlorohydrin in Examples 1 to 3 and propylene oxide in Examples 4 to 6. The cyclic acid anhydrides used are phthalic anhydride in Examples 1 to 4, endo-cis-bicyclo(2,2,1)-5-heptene-2,3-dicarboxylic anhydride in Example 5, and succinic anhydride in Example 6. The catalysts used are triethylaluminum in Examples 1 to 3 and 6, diethylzinc in Example 4, and diethylzinc and water in Example 6.
Among Examples of the Gazette, Example 6 in which an aliphatic polyester is formed comprises performing a ring-opening copolymerization reaction of propylene oxide with succinic anhydride. However, the polyester formed in this Example 6 is inferior in that its melting point is low since the cyclic ether used is propylene oxide.
If there is performed the ring-opening copolymerization, a reaction of a cyclic acid anhydride containing succinic anhydride as a main component with a cyclic ether containing ethylene oxide as a main component, in the polymerization tube according to the process of the above-mentioned Gazette, there is formed a polymer having a melting point lower than a polymer obtained from a polycondensation reaction of a dicarboxylic acid containing succinic acid as a main component with a glycol containing ethylene glycol as a main component. The reason is as follows. In the polycondensation reaction, dicarboxylic acid molecules and glycol molecules always bond one by one alternately. In contrast, in the ring-opening copolymerization reaction, not only cyclic acid anhydride molecules and cyclic ether molecules bond one by one alternately, but also the cyclic ether molecules bond with each other to form polyether chains.